Photonic integrated circuits (PICs), as are commonly used, for example, in optical routers and switches, generally include input/output couplers for optically connecting the PICs to optical fibers or other off-chip optical connectors. For example, planar waveguides in a PIC may end in grating couplers that can surface-couple light (via the top surface of the photonic chip) into out-of-plane optical fibers. Alternatively, the waveguides may be edge-coupled to fibers at side faces of the photonic chip. To ensure efficient coupling of light between the optical connector (such as an optical fiber or fiber ribbon) and the PIC, the communication channel(s) of the optical connector (e.g., the individual fibers of a fiber ribbon) need to be precisely aligned with the input/output coupler(s) of the PIC. For multi-mode optical signals, where alignment accuracies within 2 μm are sufficient, alignment can be achieved visually based, for instance, on fiducial markers placed on the photonic chip at accurately known positions relative to the input/output couplers. Efficient single-mode coupling, however, relies on accuracies within 1 μm or less, which exceeds the performance of visual alignment. Therefore, single-mode fibers or other optical connectors are usually aligned to the PIC actively.
During active alignment, light may be coupled from the optical connector into an input coupler of the PIC and measured by a detector of the PIC, or, alternatively, light generated by an on-chip light source may be coupled from an output coupler of the PIC into the optical connector and measured by an off-chip detector. Either way, by maximizing the intensity of the detected signal as the optical connector is wiggled about the approximate location of the input/output coupler of the PIC, the alignment can be optimized. Active alignment is, however, time-consuming because it involves powering up the PIC (to enable use of the on-chip light source or detector), which entails time to establish the requisite electrical connections and time waiting for the PIC to reach thermal and optical stability. Accordingly, an alternative method for single-mode alignment is desirable.